Add butter to a large saucepan. Melt the butter over medium heat. Add in the marshmallows. Stir until the marshmallows are completely melted. Remove pan from the heat and add in the vanilla, cinnamon, and nutmeg. Stir to evenly combine.

In a large mixing bowl, combine the apple, oats, and Rice Krispies. Stir to evenly distribute all the ingredients. Pour in the marshmallow mixture and carefully stir until every little bit is coated.

Butter a 8x8-inch baking pan. Spread a little butter on your hands too. It will allow you to pat down the mixture without it sticking to your hands. Press the mixture into the baking pan, making sure it is in every corner and the top is even.

Let the treats cool for fifteen minutes before slicing and topping with a salted caramel sauce.